Despite searing temperatures, Mercury, the planet closest to the sun, has ice and frozen organic materials inside permanently shadowed craters in its north pole, according to Nasa scientists.

Earth-based telescopes have been compiling evidence for ice on Mercury for 20 years, but the finding of organics was a surprise, say researchers with Nasa's Messenger spacecraft, the first probe to orbit Mercury.

Both ice and organic materials, which are similar to tar or coal, were believed to have been delivered millions of years ago by comets and asteroids crashing into the planet.

"It's not something we expected to see, but then of course you realise it kind of makes sense because we see this in other places," such as icy bodies in the outer solar system and in the nuclei of comets, planetary scientist David Paige, with the University of California, Los Angeles, said.

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"The explanation that seems to fit all the data is that it's organic material," said Sean Solomon, lead Messenger scientist with Columbia University in New York.

"It's not just a crazy hypothesis. No one has got anything else that seems to fit all the observations better," Mr Paige added.

Scientists believe the organic material, which is about twice as dark as most of Mercury's surface, was mixed in with comet- or asteroid-delivered ice eons ago.

The ice vaporized, then re-solidified where it was colder, leaving dark deposits on the surface. Radar imagery shows the dark patches subside at the coldest parts of the crater, where ice can exist on the surface.

The areas where the dark patches are seen are not cold enough for surface ice without the overlying layer of what is believed to be organics.

Scientists don't believe Mercury is or was suitable for ancient life, but the discovery of organics on an inner planet of the solar system may shed light on how life got started on Earth and how life may evolve on planets beyond the solar system.